The pathophysiology of fragile X (and what it teaches us about synapses)

Abstract

Fragile X is the most common known inherited cause of intellectual disability and autism, and it typically results from transcriptional silencing of FMR1 and loss of the encoded protein, FMRP (fragile X mental retardation protein). FMRP is an mRNA-binding protein that functions at many synapses to inhibit local translation stimulated by metabotropic glutamate receptors (mGluRs) 1 and 5. Recent studies on the biology of FMRP and the signaling pathways downstream of mGluR1/5 have yielded deeper insight into how synaptic protein synthesis and plasticity are regulated by experience. This new knowledge has also suggested ways that altered signaling and synaptic function can be corrected in fragile X, and human clinical trials based on this information are under way.

Figure 1

Fulfilling the promise of molecular medicine in FX. Martin & Bell (1943) described a group of patients characterized by a common set of features that included intellectual disability and social withdrawal. The causative gene mutation was discovered in 1991 (Pieretti et al. 1991, Verkerk et al. 1991). The FMR1 gene on the X chromosome is silenced, and the protein FMRP is not produced. Shortly thereafter, the Fmr1-KO mouse model was generated (Dutch-Belgian Fragile X Consort. 1994) and has been intensively studied by neurobiologists interested both in the disease and FMRP. In 2002, it was discovered that a form of synaptic plasticity—mGluR LTD—was exaggerated in the Fmr1 KO mouse (Huber et al. 2002). This led to the mGluR theory of fragile X (Bear et al. 2004), which posits that many symptoms of the disease are due to exaggerated responses to activation of mGluR5. The theory was definitively validated in 2007 with the demonstration that multiple FX phenotypes are corrected in the Fmr1-KO mouse by genetic reduction of mGluR5 protein production (Dolen et al. 2007). In addition, numerous animal studies showed that pharmacological inhibition of mGluR5 ameliorates FX mutant phenotypes. In 2009, inhibitors of mGluR5 entered into human phase 2 trials (http://clinicaltrials.gov). If successful, these trials will represent the first pharmacological treatment for a neurobehavioral disorder that was developed from the bottom up: from gene discovery to pathophysiology in animals to novel therapeutics in humans. Abbreviations: CGG, cytosine-guanine-guanine; FMRP, fragile X mental retardation protein; FX, fragile X; mGluR5, metabotropic glutamate receptor 5; KO, knockout; LTD, long-term synaptic depression. Image courtesy of FRAXA Research Foundation, with permission.

Figure 2

Functional domains of FMRP. Human FMRP, a 632 amino acid polypeptide (gray bar), has a nuclear localization signal (NLS; light blue), two K-homology domains (KH1 and KH2; orange), an RGG (arginine-glycine-glycine) box (dark blue), and a nuclear export sequence (NES; red). R138Q and I304N are naturally occurring mutations in patients with developmental delay and a severe form of FX, respectively. I304N abolishes polyribosome association. S500 is a major site of phosphorylation. Abbreviations: N, amino terminus; C, carboxy terminus; FMRP, fragile x mental retardation protein.

Figure 3

Excessive protein synthesis in the hippocampus of Fmr1-KO mice. Translation rates in the hippocampus measured by metabolic labeling in vitro (a,b) and in vivo (c) confirm that FMRP functions to negatively regulate protein synthesis in neurons. (a) Basal protein synthesis is significantly increased in Fmr1-KO hippocampal slices compared to control WT. Although there is no effect of reducing mGluR5 by 50% in Grm5 heterozygous mice (HT), crossing these mice with Fmr1-KO mice (CR) is sufficient to correct the excessive protein synthesis (modified from Dolen et al. 2007). (b) Excessive protein synthesis in Fmr1-KO hippocampal slices is restored to normal levels by acute treatment with an mGluR5 inhibitor (MPEP), demonstrating it occurs downstream of constitutive mGluR5 activity (modified from Osterweil et al. 2010). (c) Nissl-stained coronal sections (top panel) and their corresponding pseudocolored autoradiograms (middle and lower panels) show quantitative increases in translation rates throughout the hippocampus of 6-month-old Fmr1-KO mice in vivo (lower panel) compared with WT controls (middle panel). Images courtesy of C.B. Smith (Qin et al. 2005). Hot colors represent higher rates of synthesis. Abbreviations: FMRP, fragile X mental retardation protein; KO, knockout; mGluR, metabotropic glutamate receptor; MPEP, 2-methyl-6-(phenylethynyl)-pyridine; WT, wild type.

Figure 4

FMRP regulates mRNA translation. FMRP (red ovals) can be found bound to coding regions of mRNA in association with stalled ribosomes [complexes of 40S (small gray ovals) and 60S (large gray ovals) ribosomal subunits] and bound to 3′UTRs in association with inhibitory components of the initiation machinery (indicated by an inhibitory line). Data currently suggest that FMRP normally represses translation by stalling the elongation of actively translating ribosomes and by blocking the initiation of ribosome assembly. Loss of FMRP (as in fragile X) removes both of these inhibitory associations and leads to increased protein synthesis. Curly blue lines represent ribosomally synthesized polypeptide chains that lengthen as translation proceeds. Small arrows indicate active movement. Abbreviations: AUG, initiation codon; FMRP, fragile X mental retardation protein; m7G, 7-methylguanylate cap; ON, translation on; OFF, translation off; UAG, termination codon; 3′UTR, 3 prime-end untranslated region.

Figure 5

mGluR1/5 signaling pathways relevant to protein synthesis. Glutamate binding to Gp1 mGluRs activates three main pathways that couple the receptors to translational regulation: (a) the PLC/calcium-calmodulin pathway (orange ovals), (b) the mTOR pathway (blue ovals), and (c) the ERK pathway (green ovals). See main text for details. Key translational regulatory components implicated in these pathways are shown in brown. mGluR1/5 may also inhibit FMRP (red oval) function to regulate translation through a fourth pathway requiring stimulation of PP2A (yellow oval). Question marks indicate undetermined associations. Arrows indicate a positive consequence on downstream components; perpendicular lines indicate an inhibitory consequence. Abbreviations: [Ca2+]i, calcium release from intracellular stores; CaM, calmodulin; ERK, extracellular signal–regulated kinase; FMRP, fragile X mental retardation protein; (Gαq, Gβ, Gγ), heterotrimeric G proteins; InsP3, inositol-1,4,5-triphosphate (InsP3); mGluR, metabotropic glutamate receptor; mTOR, mammalian target of rapamycin; PtdIns, phosphoinositides; PLC, phospholipase C; PP2A, protein phosphatase 2A; Raptor, regulatory-associated protein of mTOR.

Figure 6

Schema for coupling mGluR5 to FMRP-regulated protein synthesis. Several lines of evidence suggest that mGluR5 couples to FMRP-regulated protein synthesis through multiple pathways. (a) Activation of mGluR5 directly stimulates mRNA translation through the ERK signaling pathway. (b) Additionally, activation of mGluR5 can trigger dephosphorylation of FMRP by PP2A, which derepresses translation. (c) FMRP is rapidly synthesized in response to mGluR5 activation, providing a negative-feedback loop to turn off protein synthesis. (d) Several FMRP target proteins are known components of mGluR5 signaling pathways, suggesting that positive feedback may occur, particularly in the context of FX. Abbreviations: ERK, extracellular signal–regulated kinase; FMRP, fragile X mental retardation protein; FX, fragile X; mGluR, metabotropic glutamate receptor; PP2A, protein phosphatase 2A.

Figure 7

The two-pool hypothesis. A model to account for the opposing mGluR5 responses detected in the Tsc2+/− and Fmr1-KO mice proposes that activation of mGluR5 stimulates the translation of a pool of mRNAs (Pool I), through ERK- and FMRP-dependent pathways, that are in competition for the translational machinery with a second pool of mRNAs (Pool II) that are regulated by mTOR activation. Current data suggest that mRNAs translated in Pool I may comprise the proteins required to stabilize LTD (LTD proteins), whereas mRNAs within Pool II stabilize LTP (LTP proteins). Consistent with this proposal, derepression of Pool I in FX causes excessive LTD, whereas derepression of Pool II in TSC causes enhanced LTP. Arrows indicate a positive consequence on downstream components; perpendicular lines indicate an inhibitory consequence. Abbreviations: ERK, extracellular signal–regulated kinase; FMRP, fragile X mental retardation protein; FX, fragile X; KO, knockout; LTD, long-term synaptic depression; LTP, long-term synaptic potentiation; mGluR, metabotropic glutamate receptor; mTOR, mammalian target of rapamycin; TSC, tuberous sclerosis complex.

Figure 8

Mutations causing monogenic autism define an axis of synaptic pathophysiology. Recent data suggest that proper synaptic function requires an optimal level of mGluR-regulated protein synthesis and that deviations in either direction can produce similar impairments in cognitive function (Auerbach et al. 2011). Two types of monogenic autism, TSC and FXS, lie on opposite ends of this spectrum and, correspondingly, show reduced and increased protein synthesis rates, and respond to opposite alterations in mGluR5 activation (PAM and NAM, respectively). Abbreviations: FXS, fragile X syndrome; mGluR, metabotropic glutamate receptor; NAM, negative allosteric modulator; PAM, positive allosteric modulator; TSC, tuberous sclerosis complex; WT, wild type.